THE SOIL PARTICLE 



71 



pie. The method, however, is slow, as the time necessary for 

 each subsidence of the finer particles is very great and the 

 number of individual subsidences is large. As a consequence, 

 it has been superseded by methods that utilize centrifugal 

 force for the finer separations, while retaining gravity for 

 removing the various grades of sand. 



• : I ' * " ' •  • r "• * ' i .* * 



• 



Fig. 14. — Diagram showing the relative sizes of soil particles as they 

 appear under a microscope with eye-piece micrometer. Particles 

 one space or less in diameter are clay; from one space to ten, silt 

 and above ten spaces, very fine sand. 



40. Bureau of Soils centrifugal analysis. — Of the cen- 

 trifugal methods used in mechanical analysis that employed 

 by the United States Bureau of Soils 1 is the most successful. 

 A five-gram sample of well-pulverized soil is put into a shaker 

 bottle of about 250 cubic centimeters capacity. This bottle 

 is filled about two-thirds full of water so that in shaking the 

 disintegrating force of the liquid may be utilized. A few 



1 Fletcher, C. C. and Bryan, H., Modifications of the Method of Soil 

 Analysis; U. S. Dept. Agr., Bur. Soils, Bui. 84, 1912. 



